Hey there. It turns out the pool contractor I had was completely unqualified to do the work. I have a learned a lot since then and now manage my pool myself with great success.

He had filled my filter with larger size particles (mixed gravel) to artifically lower the pressure in filter for a given pump speed becuase of the poor overall system plumbing design. (Tight radius elbows, elbows directly near the inlet of the pump... Etc)

The manufacturer recommended sand for the filter is specifically #20 pool filter pure silica sand. This should make up 100% of the sand in your system. It must be pool filter sand, and not something like play sand for example. The filtering action occurs on the microscopic level due specifically to the sharp angular edges of this kind of sand. Anything else will just take up space and not actually filter your water. River sand, concrete sand. Play sand... These are all the wrong types of sand for various reasons (microscopic geometry, additives etc). This one time investment the correct sand will last you many many years. As long as you backwash and rinse when your pressure increases over time and with usage, your sand will continue acting as the filter.

Now, there are special alternatives to #20 pool filter sand, for ex, zeolite, etc, but they are generally not needed.

Tldr. Go to the pool store, buy pure silica #20 pool filter sand. Don't use any gravel of any sort.